This invention relates to an improvement in a method of treating a workpiece with electron beams and apparatus therefor.
An apparatus for treating a workpiece with electron beams an exemplified by an electron beam welder includes a vacuum chamber for welding in which the workpiece is placed and a vacuum chamber for an electron beam gun in which an electron beam generator is disposed and which is connected air-tight to the vacuum chamber for welding, and the electron beams are emitted towards the workpiece through an aperture bored on the exterior wall of the vacuum chamber for welding. Such an apparatus is disclosed, for example, in U.S. Pat. No. 3,617,686 granted to Dietrich on Nov. 2, 1971.
In the apparatus of the above-described kind, it is generally necessary to evacuate the vacuum chamber for welding and the vacuum chamber for an electron beam gun by means of a vacuum pump and to maintain them constantly at high vacuum of about 1.times.10.sup.-4 Torr and about 1.times.10.sup.-5 .about.1.times.10.sup.-6 Torr, respectively. However, gas components contained in the workpiece such as hydrogen and oxygen molecules of H.sub.2 gas and O.sub.2 gas and molecules of the workpiece converted into the metal vapor are emitted from the surface of the workpiece during welding and eventually lower the vacuum of both vaccum chambers. If a great number of molecules enter the vacuum chamber for an electron beam gun, insulation in the proximity of anode and cathode lowers whereby micro-discharge and flash-over discharge occur. The micro-discharge results in the formation of blow-holes at the weld zone while the flash-over discharge leads to the formation of recesses of the surface beads, blow-holes and sags. If the flash-over discharge occurs, furthermore, an excess current relay operates to stop the operation of the apparatus in order to protect the same.
These discharge phenomena take place frequently when the thickness of the workpiece exceeds 100 mm because the number of the gas molecules and that of the metal vapor molecules formed during welding increase with an increasing thickness of the workpiece. When a 100 mm-thick killed steel is welded, for example, the micro-discharge occurs 1-5 times within 10 minutes and the flash-over discharge occurs about once in 20 minutes. The vacuum in this instance is 1.times.10.sup.-3 Torr near the aperture bored on the exterior wall of the vacuum chamber for welding, 1.times.10.sup.-2 Torr near the workpiece and 10.sup.-5 .about.10.sup.-6 Torr inside the vacuum chamber for an electron beam gun. In this case, the pressure difference between the portion near the aperture of the vacuum chamber for welding and the vacuum chamber for an electron beam gun reaches as large as 10.sup.2 -10.sup.3 Torr so that the gas and metal molecules are apt to be sucked into the vacuum chamber for an electron beam gun. In consequence, the molecules emitted from the surface of the workpiece enter the vacuum chamber for an electron beam gun and lower instantaneously the vauum near the anode and cathode down to 10.sup.-3 -10.sup.-4 Torr, thereby causing the discharge phenomena. Because of these discharge phenomena, it has been difficult in the conventional apparatus to weld, cut or bore a workpiece of a thickness of 100 mm or more by means of the electron beams.